This invention relates generally to television signal scrambling systems and particularly to an improved television scrambling system utilizing RF suppression of sync and carrier phase inversion. Such a system is fully described in U.S. Pat. No. 4,706,283, which is incorporated by reference herein, and which is produced by Zenith Electronics Corporation under its trademark "PM".
The PM television scrambling system described in the above patent incorporates three filters that are selectively used, in alternating pairs, to translate the video modulated IF signal. Data is sent in the horizontal intervals of the television signal by the expedient of using two different time intervals to represent the two states of a data bit. The "A" filter imparts substantially no amplitude change or phase reversal to the signal, the "B" filter imparts a 6 dB attenuation and a 180.degree. phase reversal and the "C" filter imparts a 10 dB attenuation and a 180.degree. phase reversal. Selectively switching between the "A" and "B" filters and the "A" and "C" filters, during the horizontal intervals, generates a scrambled signal. Descrambling in the television receiver is accomplished by switching three filters A, B and C into the received signal path that are complementary to their counterpart filters "A", "B" and "C" in the transmitter.
The above-described system has occasionally been compromised by so-called "pirate decoders" Enhancements have been added from time to time to thwart signal piracy and have generally been successful For example, in addition to the mode control signal, a framing code signal is sent in the data for determining which filter pair (A/B or A/C) is to be used in the decoder to recover the scrambled signal. The mode control signal consists of two bits and the framing code signal is either 18 or 24 microseconds in width. In practice, these signals are cycled periodically in an effort to confuse a would-be pirate. Because of the relative slowness of the changes in switching however (due in part to minimize signal artifacts from disturbing the picture recovered by authorized decoders) a determined pirate could still compromise the decoder sufficiently to produce a viewable display. The present invention is directed toward improving the anti-piracy attributes of the PM scrambling system.
The most common method of signal piracy involving the above-described scrambling system incorporates a simple modification of the decoder to cause it to descramble continuously using A/C filter switching. Obviously; when the headend encoder is using A/C switching, a fully descrambled picture is obtained. When the headend encoder is using A/B filter switching; often a watchable picture is obtained, the degree of viewability being determined in part by the particular type of television receiver that is connected to the decoder. Some pirate decoders include a toggle switch for controlling the changeover from A/C switching to A/B switching. As mentioned above, because the elapsed time between switching is relatively long; such a pirate decoder can often be effective. The system enhancement of the invention will result in complete loss of vertical sync in the descrambled signal output of such pirate decoders.